Stability is one of the most important factors which determines whether a compound or a mixture of compounds can be developed into a therapeutically useful pharmaceutical product. When mixing different ingredients in a pharmaceutical formulation there exists the possibility of interactions taking place between the components. In addition, each component may have different degradation characteristics.
Formoterol is a highly potent and selective β2-agonist with a long duration of action when inhaled. Compared to other β-adrenergic compounds it has a unique chemical structure with a formamido group substituted on the benzene ring. It has two asymmetric carbon atoms in the molecule making four stereoisomers possible. Most studies, clinical and preclinical, appear to have been performed with the fumarate (as dihydrate) of the enantiomeric mixture designed R;R+S;S. The R;R enantiomer is the most potent of the four enantiomers.
The stability profile of the drug formoterol (mainly as fumarate dihydrate) has been evaluated by investigating the influence of variables such as storage time, temperature, relative humidity, light and pH on the content of formoterol and determining the amount of chromatographic impurities. Formoterol (as fumarate dihydrate) has been demonstrated to be stable under long-term storage even at high temperatures and high relative humidities.
However, the chemical structure of formoterol makes the molecule prone to chemical degradation when in contact with e.g. a reactive species like an aldehyde or under stress conditions e.g. a milling process.
Potent drugs for administration by inhalation are generally formulated in association with carriers/diluents such as lactose to facilitate accurate dosing from an inhaler. These formulations have generally consisted of coarse particles of a carrier together with fine particles of the drug(s), optionally together with small particles of carrier/diluent, which combination is generally known as an ordered mixture. An alternative to such a formulation is to agglomerate the small particles of the drug(s) and the carrier/diluent to agglomerates.
Formoterol (as fumarate dihydrate) as well as a carbohydrate such as lactose (preferably as the monohydrate) are very stable compounds individually, but degradation products are formed when the two compounds are mixed. A mixture of formoterol fumarate dihydrate and lactose monohydrate can be regarded as a three component system composed of formoterol fumarate, lactose and water. By sorption of water a saturated aqueous lactose solution is formed at the surface of the powder mixture. A certain amount of formoterol fumarate dissolves in this aqueous solution and is thereby susceptible to degradation. Therefore, the relative humidity, as well as the storage temperature, will influence the stability of the powder mixture.
When adding a third ingredient in the mixture the formation of degradation products would be expected to be higher due to the complexity and the possibility for many degradation processes. It would therefore be desirable to develop a formulation with good stability in spite of the complex mixture of compounds having reactive chemical functions such as an amine (formoterol), formamide (formoterol), carbohydrate (e.g. lactose) and a keto function (glucocorticosteroid). The presence of hydrates (formoterol fumarate dihydrate, lactose monohydrate) will make it even more complex.